Applicant has previously submitted a patent application about syringe (U.S. patent application Ser. No. 11/349,189) which includes a barrel A10, a push rod A20, a driving module A30 and a needle mount A40. The barrel A10 has a hollow passage A11 inside and is coupled with a barrel cover A12. The push rod A20 has one end wedged with a pad A24 on the periphery to form a close contact with the hollow passage A11 in a slidable fashion. The barrel cover A12 houses a plunger A31, the driving module A30 of a piston pump A32 and the needle mount A40 held in the piston pump A32 that has a cut surface A412. Patents have been granted for that application in Germany and Taiwan. It is widely appreciated in the medical field. The present invention aims to provide an improvement over the aforesaid patent with fewer elements, at a lower cost and to hold a needle A45 after injection.